Electrochromic devices and methods

ABSTRACT

An electrochromic device comprising a substrate, a set of electrodes disposed on or within the substrate, and a layer comprising ε-WO3 disposed in electrical communication with the set of electrodes, wherein the layer of ε-WO3 exhibits polarization switching are described. Methods of making and using the electrochromic devices are also described. The electrochromic devices are used for detecting acetone in a fluid. The observed change in color of the ε-WO3 layer can be correlated with a subject&#39;s medical condition, such as diabetes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and benefit of the filing date of,U.S. patent application Ser. No. 16/268,094, titled ElectrochromicDevices and Methods, filed Feb. 5, 2019, the entire contents of which ishereby incorporated by reference in its entirety. This application alsoclaims priority to, and benefit of the filing date of, U.S. ProvisionalApplication No. 62/626,444, filed Feb. 5, 2018, which is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government Support under Grant No. DMR1724455 awarded by the National Science Foundation. The Government hascertain rights in the invention.

FIELD

The present invention relates to electrochromic devices and methods,and, more particularly, to electrochromic devices comprising epsilonphase tungsten trioxide (ε-WO3) and methods of making and using thesame.

BACKGROUND

Electrochromic materials exhibit reversible changes in opticalproperties, such as, for example, reflectance, absorbance, and/ortransmittance in the presence of an applied voltage. Tungsten trioxide(WO3) is a known electrochromic material that exhibits a range of colorsstimulated by redox reactions induced by changes in the applied voltage.The electrochromism of WO3 can be attributed, in part, to theintercalation and deintercalation of ions respective to the WO3 crystalsduring voltage switching. While lithium ion (Li⁺) intercalationgenerally accompanies electrochromic color shifts in WO3, thecharge-balancing counter flow of electrons associated with theintercalation and deintercalation leads to a change in the electrondensity of WO3 and, thus, changes in coloration and/or other opticaleffects.

Currently, electrochromic devices that utilize WO3 require both an ionstorage layer and an electrolytic layer to induce coloration changes viathe redistribution of charges and ion flow associated with intercalationand deintercalation. Inclusion of such layers in an electrochromicdevice is problematic, however, from a cost, design, and ease ofmanufacture standpoint. Moreover, the provision of lithium inelectrochromic devices can adversely affect humans and the environment.

Accordingly, a need exists for improved electrochromic devices andmethods of making and using the same.

SUMMARY

The subject matter herein provides for improved electrochromic devicesand methods of making and using the same. Such devices and methodsadvantageously incorporate epsilon phase tungsten trioxide (ε-WO3), andare devoid of ion storage layers, electrolyte and/or electrolyticlayers, and/or lithium or other metals for promoting electrochromism.

Without being bound by theory, it is believed that the reversiblecoloration of the electrochromic devices described herein is resultantfrom the asymmetric ferroelectric properties associated with ε-WO3, suchproperties being coupled with the polarization switching of the devices.Accordingly, the devices set forth herein exhibit both electrochromicand ferroelectric properties suitable for use in a variety of switchingapplications. The electrochromic devices described herein are simpler,less expensive, and easier to manufacture than existing devices.Moreover, the electrochromic devices and methods of making and using thesame devices do not rely on intercalation/deintercalation of hydrogen,lithium, or other ions in the electrochromic layer.

In one aspect, an electrochromic device is provided. Such a devicecomprises at least one set of electrodes such as interdigitatedelectrodes and a layer of WO3 disposed on, over, or in contact with theelectrodes. The layer of WO3 is in electrical communication with theelectrodes, and comprises up to 100% ε-WO3, based on the total weight ofthe layer of WO3. The layer of WO3 further exhibits polarizationswitching. Additionally, a layer of WO3 described herein can exhibitasymmetric ferroelectric behavior.

In some embodiments, the layer of WO3 comprises nanoparticles of ε-WO3having an average size of up to 100 nm in at least two dimensions. Infurther embodiments, the layer of WO3 comprises nanoparticles of ε-WO3having an average size of up to 60 nm in three dimensions. Moreover, insome cases, the average size (in two or three dimensions) of particlesforming a layer of ε-WO3 described herein is no larger than theferroelectric domain size for the material.

Additionally, the layer of WO3 can comprise pure ε-WO3, doped ε-WO3, ora mixed phase of ε-WO3. In cases where the layer of WO3 is doped, it maybe doped with silicon and/or chromium.

In further embodiments, the layer of WO3 does not include and/or issubstantially devoid of intercalated ions, such as hydrogen or lithiumions. Moreover, the intercalated ions of the layer of WO3 do notdetermine a color of the layer of WO3 and/or a color change exhibited bythe layer of WO3. That is, the colorization of the layer of WO3, is notdriven by the intercalation of ions.

In other respects, the layer of WO3 is electrochromic and switchesbetween a colored state and a bleached state in response to an appliedbias between the electrodes such as between the interdigitatedelectrodes. In the colored state, for example, the layer of WO3 exhibitsa uniform or substantially uniform color distribution. Alternatively,the layer of WO3 may exhibit a non-uniform color distribution. Forexample, in certain cases, the color distribution is locally disposed ordistributed proximate to the electrodes, or is a function of thedistance from the electrodes.

Furthermore, in some cases, the layer of WO3 is disposed on a substrate.The substrate may be translucent and/or transparent. Alternatively, thesubstrate may be opaque. Exemplary substrates include or are formedfrom, for example, and without limitation, glass, ceramic, paper,plastic, or any combination thereof.

Notably, the devices set forth herein do not include and/or are devoidof an ion storage layer. Further, such devices do not include and/or aredevoid of an electrolytic layer.

Electrochromic windows, mirrors, and/or displays can comprise, consist,or consist essentially of the devices described herein. Further,non-volatile memory elements can comprise, consist, or consistessentially of the devices described herein.

In a further aspect, a method of making an electrochromic device isprovided. Such a method comprises forming a layer of WO3, wherein thelayer contains primarily ε-WO3, and placing the layer of WO3 inelectrical communication with a set of electrodes (e.g., interdigitatedelectrodes). The layer of WO3 exhibits polarization switching.Additionally, the layer of WO3 can be formed via flame spray pyrolysis,laser pyrolysis, high energy ball milling, or a rapid solidificationprocess.

The present disclosure further provides a method for acetone detectionin a fluid using the electrochromic devices described herein. Thedevices comprising ε-WO3 shows appreciable sensitivity at 10 ppm acetoneconcentration or less. The present disclosure provides a device andmethod for acetone detection in breath for diabetics.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graphical illustration of comparative X-Ray Diffraction(XRD) spectra of tungsten trioxide films.

FIGS. 2A-2D are photographs of tungsten trioxide films according toembodiments of the presently disclosed subject matter.

FIG. 3 is a graphical illustration of Raman spectra data obtained forelectrochromic glasses comprising tungsten trioxide films according toembodiments of the presently disclosed subject matter.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description, examples, and figures. Theelectrochromic devices and methods described herein, however, are notlimited to the specific embodiments presented in the detaileddescription, examples, and figures. It should be recognized that theseembodiments are merely illustrative of the principles of the presentinvention. Numerous modifications and adaptations will be readilyapparent to those of skill in the art without departing from the scopeof the invention.

In addition, all ranges disclosed herein are to be understood toencompass any and all subranges subsumed therein. For example, a statedrange of “1.0 to 10.0” should be considered to include any and allsubranges beginning with a minimum value of 1.0 or more and ending witha maximum value of 10.0 or less, e.g., 1.0 to 5.3, or 4.7 to 10.0, or3.6 to 7.9.

All ranges disclosed herein are also to be considered to include the endpoints of the range, unless expressly stated otherwise. For example, arange of “between 5 and 10” or “from 5 to 10” or “5-10” should generallybe considered to include the end points 5 and 10.

Further, when the phrase “up to” is used in connection with a givenamount or quantity, it is to be understood that the amount is at least adetectable amount or Quantity. For example, a material present in anamount “up to” a specified amount can be present from a detectableamount and up to and including the specified amount.

Electrochromic Devices

In one aspect, electrochromic devices are provided. Such devicescomprise a plurality of electrodes and least one layer of WO3 comprisingup to 100% of ε-WO3. Without being bound by theory, it is believed thatchanges in coloration and/or other optical properties exhibited by theelectrochromic devices incorporating ε-WO3 are driven by the asymmetricferroelectric properties associated with the unique crystal structure ofε-WO3. It is further believed that the electrochromic reversibility iscoupled with the polarization switching of the material. Morespecifically, in the context of the present disclosure, the term“ferroelectric material” relates to a material that exhibits, over arange of temperature, a spontaneous electric polarization that can bereversed or reoriented (known as “switching”), usually, by applicationof an electric field. Regions with different orientations of thepolarization vector can coexist within a ferroelectric sample.

Electrochromic devices set forth herein advantageously exhibit bothelectrochromic and ferroelectric properties suitable for use in avariety of switching applications. Moreover, the need for ion storagelayer(s), electrolytic layer(s), and/or intercalation of metal ions,such as lithium ions (Li⁺), in the ε-WO3 is obviated.

The layer of WO3 is in electrical communication with the plurality ofelectrodes, which, for example, can comprise or consist of at least afirst electrode and a second electrode. The first electrode and thesecond electrode can include a cathode and an anode. The layer of WO3 isoptionally disposed between the first and second electrodes.Alternately, the layer of WO3 is optionally disposed on the first andsecond electrodes. The first and second electrodes can form an electrodeset or pair configured to bias electrical current through the device ina forward or reverse direction, and, in certain cases switches betweenthe forward and reverse directions.

In certain embodiments, the first and second electrodes form a set ofinterdigitated electrodes. The term “interdigitated” as used herein,refers to, at least two complementarily-shaped electrodes, wherein“branches” or “fingers” of each electrode are disposed in an alternatingmanner, each of the alternating branches or fingers separated fromadjacent branches or fingers by an interelectrode space. In some cases,the electrodes described herein may be an interdigitated structure of atleast two working electrodes. For example, the interdigitated electrodescan be an arrangement of electrodes wherein a first working electrode(cathode) is placed next to a second working electrode (anode) in analternating manner. In some examples, the interdigitated electrodes caninclude a set of microelectrode fingers (such as from 1 micron to 1000microns or from 10 microns to 100 microns) separated by aninterelectrode space (such as from 1 micron to 1 mm or from 5 microns to50 microns). The interdigitated electrodes can contain comb, curved,square, rectangular, triangular, hexagonal, or polygonal shaped“fingers” which are arranged in an alternating fashion with respect toone another. It should be understood that other shapes of electrodes mayalso be suitable for use as interdigitated electrode. The interdigitatedelectrode arrays are particularly suitable for devices such as windowsand displays due to their planar configuration.

The layer of WO3 is disposed on or over portions of each electrode and,in some cases, directly contacts each electrode. FIG. 2 illustrates anexemplary electrochromic device. A plurality of interdigitatedelectrodes are disposed on a substrate and below the layer of WO3 of thedevice. The plurality of interdigitated electrodes can substantiallycover the substrate and/or layer of WO3. The shape of the substrateand/or layer of WO3 can be a square, rectangular, triangular, hexagonal,or polygonal shape.

In certain embodiments, the electrodes can be configured to form auniform or substantially uniform color distribution in the layer of WO3.Alternatively, the electrodes can be configured to form a non-uniformcolor distribution in the layer of WO3. The size, shape, and/oruniformity of the color distribution of the layer of WO3 can bedetermined via the size, shape, and/or placement of the electrodes beinglayered, deposited, or coated with ε-WO3, in certain cases. Notably, thelayer of WO3 exhibits polarization switching and simultaneously exhibitselectrochromic and ferroelectric behavior.

The electrodes can comprise or be formed from any material and/or haveany size, shape, property, or other characteristic not inconsistent withthe objects of the instant disclosure. In some cases, for example, theelectrodes are substantially transparent or translucent, opaque,flexible, or rigid. The electrodes can comprise or be formed from anyelectrically conductive material, such as a metal or metal oxide. Theelectrodes can, but do not have to be interdigitated. For example, insome cases, the electrodes are optionally vertically or horizontallyaligned and stacked relative to each other with the layer of WO3 beingdisposed therebetween. Any position or arrangement of electrodesrelative to each other and/or relative to the layer of WO3 iscontemplated.

Further, the layer of WO3 can comprise, consist, or essentially consistof up to 100% ε-WO3, based on the total weight of the WO3. The ε-phaseis an acentric crystal phase that exhibits polarization switching andasymmetric ferroelectric properties. The layer of WO3 can comprise atleast about 40% ε-WO3, at least about 45% ε-WO3, at least about 50%ε-WO3, at least about 55% ε-WO3, at least about 60% ε-WO3, at leastabout 70% ε-WO3, at least about 80% ε-WO3, or at least about 90% ε-WO3,based on the total weight of the WO3. Stated differently, the layer ofWO3 can comprise or consist of from about 40% to about 100% ε-WO3, fromabout 40% to about 90% ε-WO3, from about 40% to about 80% ε-WO3, fromabout 40% to about 60% ε-WO3, from about 40% to about 50% ε-WO3, fromabout 45% to about 100% ε-WO3, from about 50% to 100% ε-WO3, or fromabout 50% and 90% ε-WO3, based on the total weight of the WO3.

In some instances, the layer of WO3 comprises or is formed fromnanoparticles of ε-WO3 having an average particle size of up to 100 nmin at least two dimensions. For example, the layer of WO3 can comprisenanoparticles of ε-WO3 having an average particle size of 95 nm or less,90 nm or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 nm orless, 65 nm or less, 60 nm or less, 50 nm or less, from about 10 nm to100 nm, or from about 5 nm to 100 nm in at least two dimensions, or anysubrange therebetween. That is, the layer of WO3 can comprisenanoparticles of ε-WO3 having an average particle size of from about 10nm to 50 nm, from about 10 nm to 75 nm, from about 50 nm to 100 nm, orany other subrange not inconsistent with the instant disclosure.Additionally, ε-WO3 particle sizes that are less than about 10 nm in atleast two dimensions or greater than about 100 nm in at least twodimensions are contemplated.

Further in this regard, and in other instances, the layer of WO3comprises or is formed from nanoparticles of ε-WO3 having an averageparticle size of up to 60 nm in three dimensions. For example, the layerof WO3 can comprise nanoparticles of ε-WO3 having an average particlesize of between about 10 nm and 60 nm in at least three dimensions, orany subrange therebetween. That is, the layer of WO3 can comprise or beformed from nanoparticles of ε-WO3 having an average particle size ofbetween about 10 nm and 50 nm, between about 25 nm and 50 nm, betweenabout 40 nm and 60 nm, or any other subrange not inconsistent with theinstant disclosure.

Additionally, and in some embodiments, the layer of WO3 comprises pureε-WO3. Pure ε-WO3 as used herein refers to a layer of WO3 comprisingsubstantially 100% ε-WO3, based on the total weight of the WO3. Saidanother way, the layer comprising pure ε-WO3 does not include doped or amixed-phase WO3.

Alternatively, the layer of WO3 can comprise doped ε-WO3, or a mixedphase of ε-WO3. Where the ε-WO3 is doped, the dopants may includesilicon (Si), chromium (Cr), cerium (Ce), a vanadium phosphorus oxide(VPO), or any combination thereof. The layer of WO3 can comprise ε-WO3doped with any other compound or element not inconsistent with theobjects of the instant subject matter. The dopant such as Si or Cr canbe present in any suitable amount, based on the end use of theelectrochromic device. In certain embodiments, the dopant can be presentin an amount of up to 50% by weight, based on the total weight of thelayer of WO3. For example, the dopant can be present in an amount of 1%by weight or greater, 2% by weight or greater, 5% by weight or greater,10% by weight or greater, 15% by weight or greater, 20% by weight orgreater, 25% by weight or greater, 30% by weight or greater, 40% byweight or greater, 45% by weight or greater, from 1% to 50% by weight,from 1% to 25% by weight, from 5% to 50% by weight, from 5% to 25% byweight, from 1% to 10% by weight, based on the total weight of the layerof WO3.

Where the ε-WO3 is a mixed phase, the phases other than ε-WO3 present inthe WO3 layer can include monoclinic γ-phase, orthorhombic β-phase,tetragonal α-phase, hexagonal phase, triclinic δ-phase, cubic phase, ora mixture thereof. The synthetic method and conditions used can affectthe phase and proportion of WO3 present. Polymorphs of WO3 are known inthe art. In some examples, ε-WO3 can be present in combination withγ-phase and/or δ-phase. In certain cases, the polarization switching ofthe ferroelectric domain can be enhanced when a mix phase of ε-WO3exists, such as combination of ε-WO3 with γ-phase and/or δ-phase exists.In certain embodiments, the phases other than ε-WO3 present can be in anamount of up to 50% by weight, based on the total weight of the layer ofWO3. For example, the phases other than ε-WO3 present can be in anamount of 1% by weight or greater, 2% by weight or greater, 5% by weightor greater, 10% by weight or greater, 15% by weight or greater, 20% byweight or greater, 25% by weight or greater, 30% by weight or greater,40% by weight or greater, 45% by weight or greater, from 1% to 50% byweight, from 1% to 25% by weight, from 5% to 50% by weight, from 5% to25% by weight, from 1% to 10% by weight, based on the total weight ofthe layer of WO3.

As described herein, the layer of WO3 is electrochromic, and switchesbetween a colored state and a bleached state in response to an appliedbias between the electrodes, which, in some cases are interdigitated. Inthe colored state, current is being passed through the electrodes andthe layer of WO3 may exhibit a substantially black color, asubstantially blue color, a substantially green color, or asubstantially yellow color in some cases. In the bleached state, currentis not being passed through the electrodes and the layer of WO3 retainsits original color. For example, and in some cases, biasing theelectrodes in the forward direction causes the layer of WO3 to assume afirst coloration scheme whereas biasing the electrodes in the opposite,reverse direction causes the layer of WO3 to assume a second colorationscheme. The first coloration scheme can comprise coloration beinglocalized proximate to the first electrode and the second colorationscheme can comprise coloration being localized proximate to the secondelectrode. Biasing the current can induce coloration at a desiredelectrode. Additionally, switching the current can switch a location ofthe electrically induced or triggered coloration. As coloration in thelayer of WO3 is reversible, removal of the electrical bias will returnthe layer of WO3 to its original bleached state. In the colored state,the layer of WO3 can exhibit a uniform or substantially uniform colordistribution. Alternatively, the layer of WO3 can exhibit a non-uniformcolor distribution. The size, shape, and/or uniformity of the colordistribution can be determined via the size, shape, and/or placement ofthe electrodes being layered, deposited, or coated with ε-WO3, incertain cases.

Notably, in some embodiments, the layer of WO3 comprising ε-WO3 does notinclude intercalated ions, particularly intercalated ions of aparticular type. For example, in some instances, the crystal structureε-WO3 present in a layer described herein is devoid or otherwise notintercalated with lithium ions, or any other metal ions for effecting acolor or color change of the layer of WO3.

The electrochromic devices described herein can further comprise asubstrate. The electrodes and/or layer of WO3 can, singly or combined,be disposed on or over the substrate. In some embodiments, the substratecan be transparent or opaque. The substrate can be formed from anymaterial not inconsistent with the objects of the instant disclosure.For example, and in some cases, the substrate is formed from glass,ceramic, paper, plastic, or combinations thereof. The substrate can beflexible or rigid. The substrate can comprise any size and shape notinconsistent with the objects of the instant disclosure.

The electrochromic devices described herein can be used in windows suchas smart windows. In particular, electrochromic windows can be adjustedto control the amount of light and heat passing through them. In someembodiments, the electrochromic devices described herein can comprise orotherwise be used and incorporated in electrochromic mirrors ordisplays. In further instances, the electrochromic devices describedherein can comprise or otherwise be used and incorporated innon-volatile memory elements. The electrochromic devices are suitablefor use as electrochromic mirrors such as rearview mirrors (e.g., truckmirrors, interior and exterior mirrors for motor vehicles),architectural mirrors or specialty mirrors, like those useful inaeronautical, periscopic or dental and medical applications. Inaddition, the electrochromic devices can be used as electrochromicglazings (e.g., architectural glazings, like those useful in the home,office or other edifice; aeronautical glazings, such as those which maybe useful in aircraft; or vehicular glazings, for instance, windows,like windshields, side windows and backlights, sun roofs, sun visors orshade bands); electrochromic optically attenuating contrast filters,such as contrast enhancement filters, suitable for use in connectionwith cathode ray tube monitors and the like; electrochromic privacy orsecurity partitions; electrochromic solar panels, such as sky lights;electrochromic information displays; and electrochromic lenses and eyeglass, may also benefit from that which is described herein, especiallywhere substantially non-spectral selective coloring is desired.

In addition, the electrochromic devices can be used to provide an “ondemand display” for transparent devices such as windows, mirrors, andsuch the like. The window or mirror construction facilitates placementof displays, indicia and sensors and the like behind the mirror elementso that they may be viewed as an “on demand display.” As describedherein, the electrochromic devices can exhibit a substantially coloredstate upon the introduction of an applied potential. The coloringcapability of these devices may be continuously varied by controllingthe magnitude, duration, and polarity of the applied potentialintroduced thereto.

In further instances, the electrochromic devices described herein cancomprise or otherwise be used and incorporated in sensors, for examplesensors for detecting acetone in a fluid. Particularly, electrochromicsensors can comprise a substrate, a set of electrodes disposed on orwithin the substrate, and a layer comprising ε-WO3 disposed inelectrical communication with the set of electrodes. The electrochromicsensor is capable of providing a change in color indicating the presenceof acetone in the fluid. The electrochromic device can exhibit a limitof detection of 10 parts per million or less.

It is to be understood that the electrochromic devices described hereincan comprise, include, consist, or consist essentially of anycombination of components or features described herein.

Generally, electrochromic devices such as windows generally comprise upto seven layers of material. Three central layers (ion storage layer,ion conducting layer and electrochromic layer) are sandwiched betweentwo layers of a transparent conductor, all of which are furthersandwiched between two layers of glass or plastic. These windowsfunction as the result of transport of charged ions from an ion storagelayer, through an ion conducting layer into an electrochromic layer byapplying certain voltage. The presence of the ions in the electrochromiclayer changes its optical properties, causing it to absorb visiblelight, the result of which is to darken (“unbleach”) the window. Toreverse the process, the voltage is reversed, driving the ions in theopposite direction, out of the electrochromic layer, through the ionconducting layer, and back into the ion storage layer. As the ionsmigrate out of the electrochromic layer, it brightens (or “bleaches”),and the window becomes transparent again.

Notably, as indicated herein, the electrochromic devices describedherein do not include an ion storage layer or an ion conducting orelectrolytic layer. The electrochromic devices function due to shifts inthe negative z direction in ε-WO3 are larger than those in the positivez direction. Because of the inequality of shifts in the z direction, anet spontaneous polarization develops along the c axis of ε-WO3 which isthought to give the material ferroelectric properties. In ferroelectricmaterials, there exists a switching current that is associated with thealignment of ferroelectric domains. Physically, this switching currentcan be related to the required voltage needed to reorient and align theelectrical dipoles within the material to that of the external electricfield. Similar to electrochromism, ferroelectricity involves asustainable and reversible change of the material's electrical dipolewhen an electric field is applied to the material. In the case of ε-WO3which is both an electrochromic material as well as a ferroelectricmaterial due to its electronic band structure and crystallographicasymmetry respectively, a coloration effect under the application of anexternal electric field which is entirely reversible can be obtained.The electric field can be generated by an applied voltage of 10 V orgreater, such as 30 V or greater, 40 V or greater, 50 V or greater, 60 Vor greater, 70 V or greater, 80 V or greater, or from 30 V to 80 V. Thetime required to effect a color change such as transition from ableached state to a colored state appears can be related to the suppliedvoltage. In some instances, the time can be 30 minutes or less, 20minutes or less, 10 minutes of less, 5 minutes or less, 2 minutes orless, 1 minute or less, 30 seconds or less, 20 seconds or less, 10seconds or less, within applying a voltage to the ε-WO3 material. Thereversible coloration effect exhibited by ε-WO3 is a result of theferroelectric polarization and not due to interaction of ions.

Methods of Making Electrochromic Devices

In a further aspect, methods of making electrochromic devices areprovided. The methods set forth herein can be used to provide orfabricate any of the electrochromic devices described herein. Forexample, and in some embodiments, a method of making an electrochromicdevice comprises forming a layer of WO3 and placing the layer of WO3 inelectrical communication with a set of electrodes such that the layer ofWO3 exhibits polarization switching.

For example, the layer of WO3 can be deposited or otherwise formed on orover one or more electrodes and/or optionally on or over a substrateoverlying, underlying, or disposed adjacent to the electrodes. Theelectrodes and/or the layer of WO3 can each be disposed on or over thesubstrate, where desired. The electrodes and/or the layer of WO3 can butdo not have to contact the substrate directly.

Notably, the layer of WO3 primarily contains ε-WO3, meaning that themajority component of the WO3 layer is ε-WO3. In certain cases, thelayer of WO3 is formed via flame spray pyrolysis (FSP). The size ofparticles formed in FSP depend on ODOP (one droplet to one particle),ODMP (one droplet to multiple particles) or gas phase transition. Forhomogenous solutions, ODOP results in formation of uniform particlesizes in submicron or micron range, while ODMP and gas phase result innanosized particles. The gas phase mechanism is preferred when the WO3source compound (precursor) is combustible. For example, due to thecombustible nature of tungsten-isopropoxide, gas phase mechanism ispreferred by FSP.

Tungsten isopropoxide is combustible at 500° C., the combustion givesrise to an exothermic reaction and leads to formation of tungsten vaporsand decomposed organic precursors. The vapors are readily oxidized intoWO3 vapors, which then undergo nucleation and agglomeration. TheCr-doped powders are vaporized into W vapors at ˜600° C., thedecomposition of the precursor is an endothermic reaction, and followthe same route as W vapors generated from tungsten isopropoxidesolution.

The final size of the crystallites depends on the residence time andoperating parameters, these are precursor composition, fuel and oxidantflow rate and size of droplet and flame. A direct relation betweenoxidant flow-rate has been noticed for Cr-doped particles.

The formation of ε-WO3 particles can further be described as follows.The precursor solutions upon entering the flame lead to atomization andoxidation of W atoms. The existence of large temperature gradientprefers the formation of ε-WO3 particles. The particles undergonucleation, coagulation, and agglomeration. The particle size depends onthe residence time, which relates to the time the particles spend in thechamber upon atomization. A shorter residence time gives rise to smallparticle sizes and thus a stabilized ε-WO3 is received. The formation ofγ phase occurs once the critical crystallite size for the particles iscrossed, this critical size can be calculated using a heuristic model.In this disclosure, the relative high boiling points of iso-propanol(82.6° C.) and ethanol (78.37° C.) ensure less residence time and henceresult in formation of ε-WO3 particles. The percentage of c phase isgenerally higher in pure WO3 compared to Cr-doped WO3. The concentrationof metal ions in the isopropoxide solution may be lower compared to Crdoped solution. However, the source compound (precursor) can also play arole in formation of higher c phase content. The exothermic reactionfrom the combustion of tungsten isopropoxide gives rise to a highertemperature gradient compared to the endothermic decomposition ofammonium salt, for example. The high temperature gradient favors theformation of ε-phase by delaying the nucleation and yielding lowerresidence time.

Heat treatment, such as calcination, of the synthesized particles canlead to grain coarsening and an increase in crystallite size. In somecases, pure WO3 powders may exhibit a drop in % ε-WO3 content. Once thecrystallites for ε-phase cross the critical size barrier, atransformation into γ phase occurred.

In certain embodiments, methods of making a WO3 electrochromic materialcan include dissolving a tungsten trioxide source compound in a solventcomprising an alcohol to form a liquid feed stream. In some cases, thetungsten trioxide source compound comprises a tungsten compound whichexhibits an exothermic oxidative decomposition. For example, thetungsten trioxide source compound can comprise tungsten isopropoxide.Other examples of tungsten trioxide source compound can include anytungsten salt such as tungsten alkoxide or ammonium tungstate. Thesolvent can be a high boiling solvent such as an alcohol (e.g., ethanolor methanol).

The method can further comprise atomizing the liquid feed stream withinan oxygen-containing gas to form an aerosol followed by combusting thetungsten trioxide source compound, thereby forming the WO3electrochromic material, wherein the WO3 electrochromic materialcomprises ε-WO3 nanoparticles. As described herein, the method canfurther comprise rapid solidification of the ε-WO3 nanoparticles aftercombustion, calcining the ε-WO3 nanoparticles, or a combination thereof.The WO3 electrochromic material formed can comprise at least 40% byweight ε-WO3 nanoparticles, such as from 40% to 50% by weight ε-WO3nanoparticles, using the FSP method.

In other cases, the layer of WO3 can be formed via laser pyrolysis.Still further, and in some cases, the layer of WO3 can be formed viahigh energy ball milling or a rapid solidification process.

The methods for making the electrochromic devices can further includedepositing a layer comprising the synthesized ε-WO3 nanoparticles overan electrode. Preferably, the electrode is an interdigitated electrode.Any process not inconsistent with the objects of the instant subjectmatter can be used to form the layer of WO3, which primarily containsε-WO3, on or over the electrodes and/or other surface or substrate ofthe device.

It is to be understood that the methods of making electrochromic devicesdescribed herein can include any combination of components, features,and/or processes described herein.

Methods of Using Electrochromic Devices

In some aspects, methods for detecting acetone in a fluid using theelectrochromic devices described herein are provided. The method caninclude contacting an electrochromic device with the fluid; andobserving a change in color of the ε-WO3 layer, wherein the change incolor indicates the presence of acetone in the fluid. As describedherein, the electrochromic device can comprise a substrate, a set ofelectrodes disposed on or within the substrate, and a layer comprisingε-WO3 disposed in electrical communication with the set of electrodes.Using the methods disclosed herein, a concentration of acetone can bemeasured.

The observed or measured response of the electrochromic device to thepresence of acetone can be a color change. The acentric structure ofε-WO3 plays a role on the selective detection of acetone. In particular,the ε-WO3 is ferroelectric which has a spontaneous electric dipolemoment. The polarity comes from the displacement of tungsten atoms fromthe center of each [WO6] octahedra. Acetone has a much larger dipolemoment than any other gas. As a consequence, the interaction between theε-WO3 surface dipole and acetone molecules may be much stronger than anyother gas, leading to the observed selectivity to acetone detection.Notably, the observed or measured response of the electrochromic deviceto the presence of acetone is due to a color change and not a change inan electrical property of the ε-WO3 layer (for example, conductance orresistance).

An observed or measured concentration of acetone can be correlated witha subject's medical condition, such as diabetes, ketosis, or otherabnormal acetone concentration conditions.

The ε-WO3 ferroelectric materials described herein can further be usedin quantum information including cooling systems, energy storage units,sensing devices, and computer memory. In certain embodiments, the ε-WO3can be used for ion-based computing in quantum memory devices or forferroelectric-superconductor heterostructure to provide a high speed,coherent, nonvolatile switch in a solid state quantum computing system.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

EXAMPLES Example 1: Flame Spray Pyrolysis Synthesized ε-WO3

The ε-WO3 was fabricated using a flame spray pyrolysis device (e.g.,TETHIS NPSI0). X-ray diffraction (XRD) of the synthesized material wasperformed using a Rigaku (Ultima 3) testing apparatus. FIG. 1 showscomparative XRD spectra of the as received unheated flame spraypyrolysis material (2), the powder diffraction file of a referencesample (1), and the substrate (3). The synthesized material's XRDspectrum closely matches the reference sample, as seen in FIG. 1 .Additionally, FIG. 1 includes an inlay of a transmission electronmicroscopy (TEM) image or micrograph of the synthesized nanopowder. Highresolution transmission TEM was carried out in a Hitachi H-9500instrument. The as-synthesized powder was found to be nanocrystalline(see inset in FIG. 1 ).

The WO3 films were prepared by drop coating a mixture of flame spraypyrolysis synthesized ε-WO3 and 1-heptanol (purity 99%, Sigma Aldrich)onto a substrate of alumina (CoorsTek ADS-996) with gold interdigitatedelectrodes (Case Western EDC #102). Conductive silver paint (PELCO16062) was then applied to the contact pads to promote electricalcontact. The gold fingers were approximately 250 μm wide with anapproximately 250 μm gap in between and the overall sensing area wasabout 10 cm by 10 cm. Electrical polarization was achieved through theuse of a Keithley 6517a voltage generator.

Before applying an external voltage, the color of the film on theelectrode is originally a dark yellow-brown. While not completelytransparent, it is translucent enough to see the shape of the electrodebelow it. FIG. 2A shows an image of the electrode before testing. Aftera voltage is applied to the electrode, one can observe a dark colorationbeginning to form on the film gradually. It is interesting to note thatthis coloration does not persist through the entire film, see FIG. 2B,but is rather confined locally to the electrode with the black(−)voltage. If polarity of the voltage source is then switched, which isrepresented by a physically swapping of the black and red connectors,the coloration on the right electrode begins to vanish and thecoloration appears now on the left electrode, which now has the black(−) voltage. Upon switching the electrodes a third time, the colorationonce again disappears from the left side and reappears on the right sidewith the black (−) voltage. If the electrodes are left to sit in ambientair overnight, the coloration on the film completely disappears and thefilm returns to its original bleached state. FIG. 2A is a photograph ofthe film before testing, FIG. 2B is a photograph of the film after thefirst poling, FIG. 2C is after reverse poling, and FIG. 2D is asaturated film at +100 V.

Due to its perovskite like structure, WO3 is capable of beingintercalated by a number of different ions including gold and silver.However, when these atoms are included in the WO3 crystal structure, theWO3 structure is distorted. These distortions can be viewed andquantified through the use of Raman Spectroscopy. To further investigatethe cause of this coloration, Raman spectroscopy (Thermo Scientific DXR)was used to compare the Raman shift between the bleached and coloredsample.

FIG. 3 shows a comparative Raman plot in a colored region and twouncolored regions. Looking at FIG. 3 , one can see peaks at 143, 643,and 679 cm^(•) 1, which are the characteristic peaks of the £ phase.Aside from a change in the intensity of the black area, there is nonotable change between the three plots. Not intending to be bound bytheory, it is believed that it is not intercalation of the electrodeions into the WO3 structure which is causing this change.

Furthermore, although WO3 (in general, not the specific ε-WO3 layersdescribed herein) is a well-known electrochromic material, it typicallyrequires a solid or liquid electrolyte to promote the transfer of ionsfrom the ionic layer into the WO3. However, despite the absence of thislayer, ε-WO3 containing layers described herein still exhibit areversible electrochromic effect.

It is believed that the coloration effect observed here is a result ofthe material's ferroelectric properties and not of the anticipatedintercalation. Without being bound by theory, it is believed that theredistribution of charge that comes about because of the ion flow duringintercalation induces electrochromic behavior. Similarly, inferroelectric materials, when a device is poled, redistribution in thecrystal of the electrical dipoles is obtained. While these are twovastly different mechanisms, the overall effect, i.e., theredistribution of charge, is believed by the present inventors to be thesame. It is believed that this redistribution of electrical dipoles thatcomes about during ferroelectric domain inversion is the underlyingcause of this coloration change.

The presence of a reversible electrochromic effect on thick films offlame spray pyrolysis synthesized ε-WO3 on gold interdigitatedelectrodes is described above. Despite the absence of a solid or liquidelectrolytic layer, the system appears to undergo an oxidation andreduction style reaction that is not related to the intercalation of theelectrode material into the WO3 crystal structure.

Furthermore, while this coloration is reversible, it does not persistmore than a day suggesting that the excess electrons are redistributedeither throughout the system or with the ambient environment. Theappearance of this coloration while imaging in a scanning electronmicroscope in a very high vacuum debunks the idea that it is the surfaceadsorbents or atmospheric water forming HxWO3 tungsten bronzes. Againnot intending to be bound by theory, it is believed that the colorationchange occurs due to a redistribution of charge resulting fromferroelectric domain inversion and a redistribution of its electricaldipoles.

Devices and methods described herein have the potential to revolutionizethe design of WO3 electrochromic glass, including by eliminating theneed for both an ionic layer as well as an electrolytic layer. Benefitsof devices and methods described herein include the environmentalbenefit of no longer using lithium based tungsten bronzes. The use ofthe polarization switching properties of this polymorph of WO3 innon-volatile memory devices is also contemplated herein.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of skill in the artto which the disclosed invention belongs. Publications cited herein andthe materials for which they are cited are specifically incorporated byreference.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

What is claimed is:
 1. A method of making a tungsten trioxide (WO₃)electrochromic material, the method comprising: dissolving a tungstentrioxide source compound in a solvent comprising an alcohol to form aliquid feed stream, atomizing the liquid feed stream within anoxygen-containing gas to form an aerosol, and combusting the tungstentrioxide source compound, thereby forming the WO₃ electrochromicmaterial, wherein the WO₃ electrochromic material comprises ε-WO₃nanoparticles.
 2. The method of claim 1, wherein the tungsten trioxidesource compound comprises a tungsten compound which exhibits anexothermic oxidative decomposition.
 3. The method of claim 1, whereinthe tungsten trioxide source compound comprises tungsten isopropoxide,tungsten alkoxide, ammonium tungstate, or a combination thereof.
 4. Themethod of claim 1, further comprising rapid solidification of the ε-WO₃nanoparticles after combustion, calcining the ε-WO₃ nanoparticles, or acombination thereof.
 5. The method of claim 1, wherein the WO₃electrochromic material comprises at least 40% by weight ε-WO₃nanoparticles, such as from 40% to 50% by weight ε-WO₃ nanoparticles. 6.The method of claim 1, further comprising depositing the WO₃electrochromic material on an interdigitated electrode.
 7. A method fordetecting acetone in a fluid, comprising the steps of: contacting anelectrochromic device with the fluid, wherein the electrochromic devicecomprises: a substrate; a set of electrodes disposed on or within thesubstrate; and a layer comprising ε-WO₃ disposed in electricalcommunication with the set of electrodes; and observing a change incolor of the ε-WO₃ layer, wherein the change in color indicates thepresence of acetone in the fluid.
 8. The method of claim 7, whereinobserving the change in color comprises measuring a change in color ofthe ε-WO₃ layer upon exposure to the fluid.
 9. The method of claim 8,further comprising correlating the change in color with a concentrationof acetone in the fluid.
 10. The method of claim 8, further comprisingcorrelating the change in color with a subject's medical condition, suchas diabetes.
 11. The method of claim 7, wherein the electrochromicdevice exhibits a limit of detection of 10 parts per million or less.